1. Field of the Invention
The present invention relates to an image registration system and, more particularly, to an image registration method using a digital subtraction method of extracting vascular tracts by conducting subtractions between X ray images before and after injection of a contrast media, which can register the images with high accuracy and quickly process the registration.
2. Description of the Prior Art
A vascular tract contracting method by a digital subtraction technique is one of medical methods for diagnosing an abnormality in the vascular tracts of a human body. This method is intended to clearly extract the vascular tracts, into which a contrast media has been injected, by conducting subtractions between an X ray image (which will be shortly referred to as a "mask image") taken before a contrast media takes effect (which is referred to as "before injection of the contrast media" in the present invention) before or after the contrast media is injected into the human body, and an X ray image (which will be shortly referred to as a "live image"), which is taken after a predetermined time has elapsed after the contrast media injection (which will be shortly referred to as "after the contrast media injection").
Here, there arises a problem that positional discrepancies are caused between the aforementioned two images by bodily motions or heartbeats of a body. In order to correct those positional discrepancies, the prior art used a correcting method by polynomial approximation. In this correcting method, coordinate transformations are conducted between the aforementioned two images.
In order to register images X and Y assuming that the image X is expressed by coordinates (u, v) and that the image Y is expressed by coordinates (x, y), the former image X in the coordinates (u, v) has to be transformed into and expressed by the points on a lattice of the coordinates (x, y). For the coordinate transforming equations, therefore, there can be used polynomials of higher order that can be expressed as follows: ##EQU1## where: m designates an arbitrary integer smaller than the number of standard points which will be described hereinafter; and a.sub.ij and b.sub.ij designate coefficients.
The coefficients of the aforementioned coordinate transforming equations are determined by the method of least squares. By giving the combinations of the coordinates of the standard points (e.g., the branches of vascular tracts or the ends of bones), which are common between the aforementioned two images: EQU (u.sub.i, v.sub.i); (x.sub.i, y.sub.i) (i=1 to n),
the coefficients minimizing the value Q expressed by the following equation: ##EQU2## are computed by solving the following normal equations: ##EQU3## From this, it can be determined to what position of the coordinates (u, v) the point of the coordinates (x, y) shown in FIG. 1 corresponds.
The setting of the aforementioned standard points has been conducted by the method of manually designating the characteristic points on an image or the method of using a statistical technique such as correlation or the sequential similarity detection algorithm with reference to the points, which are arranged in the form of a lattice.
In the image registration methods thus far described, however, there arises a problem that image distortion become large because the image as a whole is uniformly transformed. Moreover, the processing of the image registration is realized by the use of software, but speedup has been desired because the processing takes a long time. For setting the standard points, furthermore, a method has been desired which can detect the characteristic points more conveniently and reliably.